Electronic thermostats are now very common devices, and allow improved accuracy in controlling heating and air conditioning equipment. Such thermostats have integral microprocessors which can apply sophisticated control algorithms to control overshoot or undershoot, and include setback and recovery functions which improve efficiency in maintaining space temperature. The elements of such a thermostat are mounted within an enclosure to protect and conceal them. These thermostats also include an internal temperature sensor which provides an analog signal encoding the temperature internal to the enclosure which the microprocessor converts to a digital value for control of space temperature.
The load is switched either by some type of solid state element or alternatively by a relay. In either case, heat is generated by the switching element while conducting load power. This heat generated by the load power switching element can affect the internal temperature of the thermostat enclosure and cause the temperature sensor to provide a signal which indicates a temperature value higher than the actual temperature external to the thermostat enclosure. One can see that this error can cause space temperature to be maintained at less than the desired value. Even worse, if the user attempts to correct for this error by setting the thermostat to a higher temperature than desired, the change in duty cycle arising from changes in heating or cooling load can change the error.
This problem has been previously recognized. U.S. Pat. No. 4,741,476 measures the switch element heat with a pair of thermistors at different distances from the switching element. The difference in the temperature indicated by the two thermistors provides a measure of the heat generated by the switching element.
U.S. Pat. No. 3,817,453 solves this problem by placing a thermistor adjacent to the switching element. The thermistor is in the bridge which controls thermostat operation, and the change in the thermistor resistance affects operation of the bridge and thereby the thermostat to reduce the effect of switching element heat.